1. Field of the Invention
The present invention relates to an imaging device.
2. Description of the Related Art
Imaging devices such as complementary metal oxide semiconductor (CMOS) linear image sensors read image data at certain rates. For example, a photodiode in each pixel accumulates charge by photoelectric conversion and the accumulated charge is transferred to a floating diffusion at certain timing, and the charge retained in the floating diffusion is read as image data. The following describes examples of pixel data read-out techniques of comparative imaging devices.
For example, Japanese Unexamined Patent Application Publication No. 2006-191236 discloses a solid-state image sensor that simultaneously resets pixel signals for all the rows before exposure, closes a mechanical shutter after a predetermined exposure period is elapsed, and sequentially reads the pixel signals for each row with the mechanical shutter being closed.
Japanese Unexamined Patent Application Publication No. 2013-106231 discloses an imaging device that reads a reset level of each floating diffusion and then reads charge transferred to the floating diffusion as a signal level, and calculates a difference between the signal level and the reset level.
WO2007/111854 discloses an imaging device that performs rolling shutter reset of a plurality of storage nodes such that the plurality of storage nodes are reset before charge is transferred from photosensors.
Japanese Unexamined Patent Application Publication No. 2011-066852 discloses a technique for the evaluation of linearity of an electric shutter before shipping of the product. In this technique, pixel data is output from each row while increasing exposure time in the arrangement order, and the linearity is evaluated from the characteristics that represent a relation between the exposure time of pixel data and an output level of the pixel data.
However, such comparative techniques does not set an individual accumulation period for each pixel group and keep the condition of the floating diffusion at a certain level at the time of starting the transfer of charge. If charge is transferred to a floating diffusion that still has a residue charge and is not in a reset state, the floating diffusion, which cannot store more charge than the storage capacity, may fail to receive all the generated charge transferred from a photodiode. In this case, charge remains in the photodiode, and the residue charge is added to newly generated charge in a next accumulation period, resulting in a difference between the actual object to be imaged and the image data that images the object.